The invention relates to a bumping process for semiconductor packaging and in particular to a solder bump structure with improved height and a method for forming the same.
Packaging is an essential step in the fabrication of integrated circuits, which protects the integrated circuits and provides a signal transmission interface for external circuits. Therefore, the development of packaging is affected by the development of integrated circuit technology and the function of electronic products.
A variety of packaging technologies have been developed, such as ball grid array (BGA), chip scale package (CSP), flip chip, and multi-chip module (MCM). In particular, flip chip is a commonly used packaging method, which employs a solder bump formed on a bonding pad for connection to the circuit board. The formation of the solder bump comprises solder ball mounting, printing, and electroplating.
FIGS. 1A to 1D illustrate a conventional method for forming a solder bump structure by electroplating. As shown in FIG. 1A, a substrate 100, such as a silicon substrate, is provided. The substrate 100 has a metal bonding pad 102 comprising, for example, aluminum or copper. A passivation layer 104, such as a silicon nitride layer, is formed overlying the substrate 100 and substantially exposes the metal bonding pad 102. A metal composite layer 106 is conformably formed on the passivation layer 104 and the exposed metal bonding pad 102, which is typically a metal stack of adhesion layer/barrier layer/wetting layer. In order to simplify the diagram, a single layer is depicted.
As shown in FIG. 1B, a dry pattern film 108 is formed on the metal composite layer 106, which has an opening 109 to expose a portion of the metal composite layer 106 overlying the metal bonding pad 102. Here, the opening region 109 is utilized in forming solder bump. Accordingly, the opening 109 is subsequently filled with a solder 110 by electroplating. The height of the solder is determined by the thickness of the dry pattern film 108.
As shown in FIG. 1C, the dry pattern film 108 is removed and the metal composite layer 106 uncovered by the solder 110 is then removed, exposing the underlying passivation layer 104. The remaining metal composite layer 106a acts as an under bump metallurgy (UBM) layer.
As shown in FIG. 1D, a reflow process is performed, such that the solder 110 forms a ball-shaped or hemiball-shaped solder bump 110a due to surface tension.
The height of the solder bump, however, affects reliability of packaging devices. As the size of the package is reduced, fatigue strength is degraded if the height of the solder bump is too low, reducing the bonding life. Moreover, during bonding the chip to the circuit board, the gap between the chip and the circuit board cannot be effectively filled with the underfill, thus forming holes therein.
Accordingly, the reliability of the flip chip method can be increased by providing higher solder bump. Generally, the methods for increasing the height of the solder bump are to increase the dry film thickness or size of the UBM layer, thereby increasing the capacity of the solder to accomplish the higher solder bump. Unfortunately, such methods may be detrimental for lithography or increase the occupied area of the chip, reducing the integration.
Taiwan Patent No. 90,117,002 discloses a method of flip chip bonding which employs double photoresist layers with different reactive spectrum to provide a higher solder bump. In this, method, however, the solder bump may collapse easily during removal of the double photoresist layers, such that the height of the solder bump cannot be effectively increased. Additionally, U.S. Pat. No. 6,299,220 discloses a solder bump structure, as shown in FIG. 2. The solder bump structure comprises a substrate 200 having a bonding pad 202 and a passivation layer 204 thereon. A barrier layer 206 is disposed between the bonding pad 202 and the solder bump 210. Here, the solder bump 210 comprises a composite material. That is, the lower portion 208 of the solder bump 210 comprises a material with a relatively higher melting point and the upper portion 209 with a relatively lower melting point. When a reflow process is performed, the lower portion 208 of the solder does not melt thus preventing collapse of the solder bump, maintaining the height thereof. In this method, however, two materials are required, complicating the fabrication and increasing the cost.